The use of glassy metal alloy strips to make resonant markers for article surveillance has grown dramatically over the past ten years. However, there are still many uses for such resonant markers which require a large number of resonant tags each having unique and discernable resonant characteristics. Examples of such uses include inventory control, baggage routing and control at airports, personnel identification and article surveillance and control. However prior methods have failed to provide a way to produce enough distinct markers to make resonant tags practical for uses requiring a large number of unique and distinguishable targets.
Resonant markers comprise strips of a glassy metal alloy ribbon which resonate, either electrically or mechanically, when subjected to an ac field. In the case of mechanically resonating markers, the resonant wave characteristics, wavelength, amplitude and output voltage profile, can be measured by applying an interrogating pulse or burst to the glassy metal alloy strip (alloy strip), and measuring the dampening resonance. Methods for altering the resonant characteristics include changing the physical parameters (dimensions) of the alloy strip or changing the chemical composition of the alloy strip. U.S. Pat. Nos. 4,510,489 and 4,510,490 disclose magnetomechanical markers which may be used in an article surveillance system. Each marker comprises a plurality of rectangular strips which are adjacent to a ferromagnetic element. These patents further disclose that the resonant frequency of the marker may be tailored by altering the composition, thermal preparation and/or physical parameters of the alloy strip.
However, the above methods are undesirable because the alterations must be effected at the point where the resonant markers are made, and because there are a relatively limited number of chemical compositions which are appropriate for resonant markers and a limited range of marker sizes which are practical. Thus, the universe of distinct resonant markers which could be produced according to the above patents is relatively small.
Methods for locally heat-treating thin amorphous alloy strips are disclosed in Japanese application No. 173148/1980. The amorphous alloy contains 70 atomic % or more Fe, and may be used as a magnetically soft electromagnetic material. The heat treatment may be induced via irradiation with a laser or electron beam, or current may be fed through a metal needle or edge.
However, the above application discloses a method for producing an amorphous alloy strip having a low iron loss to be used as a magnetically soft electromagnetic material, and neither the effects of the heat treatment on the resonant characteristics of the amorphous alloy strips nor the utility of the material for use as resonant markers was recognized.
U.S. Pat. Nos. Re. 32,427 and Re. 32,428 disclose harmonic markers comprising amorphous ferromagnetic alloy strip, and optionally one or more magnetizable elements. The magnetizable elements may be provided as a part of the strip by altering (e.g. crystallizing) a portion of the strip via spot welding, heat treatment with coherent or incoherent radiation, charged particle beams, directed flames, or heated wires. The magnetizable elements have higher coercivities than the amorphous material, and the resulting strip generates magnetic fields at frequencies which are harmonically related to the interrogating field. The addition of magnetizable elements as disclosed in the patents does not affect the fundamental mode of operation of the envisaged device; namely, the frequencies generated in the interrogating zone are harmonics of the interrogating field, and the presence of magnetizable elements does not provide the markers with individual identities and therefore does not increase the universe which can be generated.
Accordingly, an object of the present invention is to provide an encoding method that would enable simplified mass encoding which could provide a large number of alloy strips each having unique and discernable resonant characteristics. Another object is to provide a method of encoding which could be carried out at a point other than the point of manufacture of the markers. Encoding could be effected by the manufacturer of the alloy strips, the manufacturer of the resonant markers, or by the end user. Another object of the present invention is to provide resonant markers having measurably distinct resonant wave characteristics as described above, thereby increasing the universe without complicating the production process.